1. Field of the Invention
The invention relates to the management of hydrocarbon producing wells. More particularly, the invention relates to methods and apparatus for dynamically mapping the location of an oil-water interface and for predicting reservoir fluid movement and pressures under different production conditions.
2. State of the Art
In a petroleum reservoir, oil is produced through a well under pressure of gas, water, or compaction. Water may be naturally present in the reservoir displacing the oil to urge it out through the well bore. Often, water is injected into the reservoir from an injection bore located near the production bore. As oil is extracted from the well, the water moves through the porous medium of the formation closer to the well and the oil-water interface changes shape. If the location of the oil-water interface is not monitored during production, it is possible that the well will produce a mixture of oil and water. In some cases, it is possible for the well to produce more water than oil.
Well logs are a primary source of information used to map the distribution of fluids in hydrocarbon reservoirs. Because of the high electrical resistivity of hydrocarbons compared to formation water, open hole well logs of resistivity are typically used to infer water saturation, the percentage of pore volume occupied by water. As wells are typically cased with conductive steel pipe after drilling, it is not usually possible to take resistivity measurements through the casing. If a non-conductive casing is used, crosshole tomography techniques can be used to map the distribution of electrical resistivity in the reservoir volume. Measurement of fluid pressures is also used to estimate multiphase fluid flow properties (e.g. water and oil mobilities) and the location of the oil-water interface.
Previously incorporated U.S. Pat. No. 5,467,823 and U.S. Pat. No. 5,642,051 disclose methods and apparatus for monitoring a production reservoir with pressure and resistivity sensors which are permanently mounted in the production well between the casing and the borehole. The '823 patent does not specifically address the issue of monitoring the location of the oil-water interface and neither patent discloses any method for interpreting data acquired by the sensors in order to predict the location of the oil-water interface over time.
Previously incorporated U.S. Pat. No. 5,767,680 discloses a method for sensing and estimating the shape and location of oil-water interfaces in a formation traversed by a well. The method includes making time-lapse DC/AC measurements with an array of permanently deployed sensors in order to detect and estimate the change in geometry and proximity of the oil-water interface as a result of production, and therefore as a function of time. The estimation is carried out with a parametric inversion technique whereby the shape of the oil-water interface is assumed to take the form of a three-dimensional surface describable with only a few unknown parameters. A nonlinear optimization technique is used to search for the unknown parameters such that the differences between the measured data and the numerically simulated data are minimized in a least-squares fashion with concomitant hard bound physical constraints on the unknowns. The estimation procedure is robust in the presence of relatively high levels of noise and can therefore be used to anticipate deleterious water breakthroughs, as well as improve the efficiency with which the oil is produced from the reservoir.
The fundamental challenge posed in interpreting reservoir property measurements is to optimize each measurement of reservoir properties at time t by using all of the measurements acquired up until time t. This is difficult because the relationship between measurements and reservoir properties is generally complex and because measurement errors will affect subsequent interpretations of reservoir properties.